An Organic Light Emitting Diode (OLED) is a current-driven self-luminous type device, and for its unique characteristics such as self-illumination, fast response, wide viewing angle and the capability of being manufactured on a flexible substrate, an organic light-emitting display device based on the OLED is expected to become the mainstream in the field of display technology in the next years.
Each of display units on an organic light-emitting display device comprises an OLED, and the organic light-emitting display device can be divided into an active organic light-emitting display device and a passive organic light-emitting display device, wherein the active organic light-emitting display device refers to a display device in which, for each OLED, the current flowing through the OLED is controlled by a Thin Film Transistor (TFT) circuit, and the OLED and the TFT circuit for controlling the OLED constitute a pixel circuit.
A typical pixel circuit is illustrated in FIG. 1 and the pixel circuit comprises 2 TFTs, 1 capacitor and 1 OLED, wherein a switch transistor T2 transmits a data voltage on a data line to a gate of a driving transistor T1, and the driving transistor T1 converts the data voltage to a corresponding current for supplying to the OLED, wherein the current can be represented as follows:
                                                                        I                OLED                            =                            ⁢                                                1                  2                                ⁢                                                      μ                    n                                    ·                  Cox                  ·                                      W                    L                                    ·                                                            (                                              Vgs                        -                        Vth                                            )                                        2                                                                                                                          =                            ⁢                                                1                  2                                ⁢                                                      μ                    n                                    ·                  Cox                  ·                                      W                    L                                    ·                                                            (                                              Vdata                        -                        Voled                        -                        Vth                                            )                                        2                                                                                                          (        1        )            
wherein Vgs represents a potential difference between the gate and a source of the driving transistor T1, μn represents a carrier mobility, Cox represents a capacitance of gate-insulating layer, W/L represents a ratio of width to length of channel of the driving transistor T1, Vdata represents the data voltage, Voled represents an operating voltage of the OLED, and Vth represents a threshold voltage of the driving transistor T1. It can seen from the above equation that there is variation in currents flowing through the OLEDs if Vth varies in different pixel units or Vth drifts with time, thus affecting the display effect; further, the OLEDs have different operating voltages due to the ununiformity of OLEDs, which in turn results in variation in currents flowing through the OLEDs.